THEORY OF MULTI-ELECTRODE VACUUM' TUBES 45 



gain, amplification at higher frequencies and greater frequency dis- 

 crimination, it eventually became necessary to investigate the possi- 

 bilities of making changes in vacuum tubes enabling them to meet 

 these requirements more satisfactorily. Measures taken to meet this 

 situation have included improvements in the three-electrode tube that 

 reduce the effects of some of its limitations, and the development of 

 vacuum tubes having more than three electrodes. 



The purpose of this paper is to present, in simple form, the 

 physical principles underlying the characteristics and performance of 

 multi-electrode vacuum tubes. For present purposes, such tubes may 

 be defined as those having more than the three electrodes of the 

 conventional triode. The procedure will be to show that the definitions 

 of electrical tube parameters applicable to triodes are, with certain 

 modifications in their interpretation, also applicable to tubes having 

 more than three electrodes; and, utilizing the theory of the triode, to 

 analyze the characteristics of a few typical multi-electrode structures 

 that illustrate the types of characteristics found in many such tubes. 

 No attempt is made to present new material in the paper or to discuss 

 in detail the many different types of multi-electrode tubes now in use. 

 The author has attempted to present the subject from the viewpoint of 

 those readers who have a satisfactory understanding of the physical 

 principles, characteristics, and operation of the triode, but who do not 

 have a similarly clear analysis available for the more complex structures. 



Multi-electrode tubes may be divided conveniently into two classes. 

 In the first class are those the purpose of which is to perform some 

 function that cannot be performed readily by a triode, or which 

 perform some function better by reason of the elimination or reduction 

 of some limitation in triodes. The second class includes those struc- 

 tures in which additional electrodes are introduced to permit them to 

 perform simultaneously more than one function, or to permit them to 

 function in two or more ways, depending on the voltages applied to the 

 various electrodes and on the manner of their operation. This paper 

 will deal exclusively with typical structures of the first class. 



Fundamental Definitions and Tube Equations 



Regardless of the type of multi-electrode tube considered, the space 

 current to any electrode may be expressed as some function of the 

 voltages applied to the various electrodes. However, in the operation 

 of any multi-electrode device, or any section of such a device per- 

 forming a single function, one usually is concerned with variations in 

 the voltages and corresponding currents of only two of the electrodes, 

 the other electrodes being maintained at fixed potentials. One of these 



